The present invention relates to steering systems for land vehicles. Specifically, the present invention relates to such a steering system in which more than one axle has steering capabilities.
With very few exceptions, all known land vehicles have at least two transverse axles on which wheels are located for supporting the vehicle and allowing it to move forward. Typically, the wheel or wheels supported by the front axle are steerable -that is, they are permitted to rotate on a steering axis which extends in a direction having a large vertical component. Thus, the front wheel or wheels are permitted to steer left or right, thus causing the vehicle as a whole to turn left or right.
Recently, all-wheel steering systems have been produced to improve maneuverability and stability while driving. In such systems, the rear wheel or wheels are steered in concert with the front wheel or wheels. In systems having more than two axles, all the wheels are steered together in concert. For purposes of this disclosure, the term, xe2x80x9cfront steerxe2x80x9d will refer to steering the one or more wheels to the front of a midpoint of the vehicle while the term, xe2x80x9crear steerxe2x80x9d will refer to steering the one or more wheels to the rear of a midpoint of the vehicle, it being understood that while most vehicles have a single front axle and a single rear axle, many other combinations are possible depending on the design of the vehicle. While two wheeled vehicles are included by these definitions, as a practical matter, vehicles will generally have four or more wheels, although two and three wheeled-vehicles having all-wheel steering are certainly possible.
Determining the ideal or optimum amount of rear steering with respect to front steering has proven to be a vexing issue. One way to define the amount of rear steering is as a fraction of the front steering. Prior art all-wheel steering systems sometimes provide for negative rear steering at low speeds and positive steering at high speeds. That is, the rear-steer to front-steer ratio at low speeds is negative, causing the rear wheels to turn in an opposite direction than the front wheels, while at high speeds, the rear-steer to front-steer steering ratio is positive, and so the rear wheels turn in the same direction as the front wheels.
By steering the rear wheels in an opposite direction from the front wheels at low speeds, improved maneuverability is achieved. However there is a drawback that the rear end of an all-wheel vehicle will swing wide, i.e., go in the opposite direction as intended. While this does provide an advantage for particularly long vehicles or vehicles towing a trailer by preventing the back end from clipping the corner apex, it can be problematic when the back end unexpectedly swings wide when pulling out of tight spaces such as from gas-pumps or parking spaces. It has also been found that at high speeds, the optimum ratio for rear steering amount is greater when towing a trailer than when not towing a trailer, the optimum ratio being determined by such factors as responsiveness. See xe2x80x9cBenefits of Four-Wheel-Steering on Trailering Performancexe2x80x9d by Charles R. Worner, incorporated herein by reference.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by an all-wheel steering system for a vehicle comprising a controller having a first input for receiving a signal from a hand wheel position sensor, a second input for receiving a signal from a vehicle speed sensor, and a third input for receiving a signal that varies depending upon whether a trailer is hitched to said vehicle. The controller generates an output for controlling a rear wheel steering actuator. The output varies as a function of the first, second, and third inputs such that when a trailer is hitched to said vehicle the out-of-phase rear steer amount at low speeds is reduced and the in-phase rear steering amount at high speeds is increased.